Make the collection API parameters stable

[obulat]

Problem

#3887 adds some unstable__ query parameters for the Additional search views project.

Description

Remove the unstable__ prefix from unstable__collection and unstable__tag. Update the documentation to always show the full description for collection, tag, source and creator parameters.
Updated: this is a single-step change.

Additional context

Blocked until the project is launched.

Alternatives

This change could be implemented in 3 steps:

• make the API accept both unstable__ and stable variants (i.e., unstable__collection and collection)
• update the frontend code to use only the stable parameter names (collection and tag)
• remove the unstable__ parameters from the API.
  But it's easier to make the API change in one step, and make transition smooth in the frontend.

[sarayourfriend]

I agree with the approach you've described. The only alternative is to flip the first two steps (sort of) and have the frontend send both, until the API moves to only accept the stable version, then start only sending one from the frontend. Depending on the complexity of either, that could be preferrable. Handing two names for the same parameter might be slightly more complex in DRF serializers than e.g., updating the frontend code to temporarily send the parameter with both names.

[obulat]

I agree with the approach you've described. The only alternative is to flip the first two steps (sort of) and have the frontend send both, until the API moves to only accept the stable version, then start only sending one from the frontend. Depending on the complexity of either, that could be preferrable. Handing two names for the same parameter might be slightly more complex in DRF serializers than e.g., updating the frontend code to temporarily send the parameter with both names.

That's a great idea! I also think that adding an extra parameter to the frontend is easier, than adding serializers.